What exactly was the mortality rate of standard bearers? Was it a prestigious position, or was it more like the Harrenhal of military positions? I can’t imagine that it was very easy to fight while holding an ungainly banner.

Very high, hence the custom that if the standard bearer falls, someone else picks it up and becomes the new standard bearer. 

And yes, it was both. Standard bearers were selected from the bravest soldiers in the army (since they needed to be willing to run into danger while being less able to fight) and the strongest (because they were fighting one-handed often). 

And there was a very high mortality rate as suggested above. But just to clarify, standard bearers could and did plant the flag in order to free up a hand if and when they needed to fight; these were brave men, not idiots.